Band filter



y 1943- J. SCHWARTZ 2,318,531

- BAND FILTER Filed Sept. 7, 1940 Patented May 4, 1943 BAND FILTER Jean Schwartz, Baden, Switzerland, assignor, by

mesne assignments, to Patelhold Patentverwertungs & Elektro-Holding A.-G., Glarus, Switzerland, a joint stock company of Switzerland Application September 7, 19d0,,Serial No. 355,873

In Switzerland September 11, 1939 7 Claims.

It is known to construct band filters consisting of three oscillatory circuits coupled together. In these known filters the mutual coupling of the oscillatory circuits is about equally close and can be either of a capacitive or inductive nature. In the known types of three-stage band filters an alteration in the band width is accompanied by difficulties and can only be undertaken within certain limits. Furthermore the relative band width, that is the band width with reference to the average frequency transmitted, is necessarily comparatively small if resonance curves with two accentuated outside maxima are to be avoided. The usual three-stage band filters are also difficult to adjust as to band width because the couplings between the three systems are very can also be replacedby a low value resistance connected in series with Z3 or 03. Since such series resistances can only amount to a few ohms and at the same time should be easily regulated,

sensitive and vary with and/or affect the reactances of the three coupled circuits.

The object of the present invention is therefore to construct a band filter which enables the above mentioned disadvantages to be avoided and at the same time possesses various favourable properties which are to be described later. In a band filter having three oscillatory circuits thisis achieved according to the invention by arranging two oscillatory circuits to form a twocircuit band filter with close coupling, this latter filter being only loosely coupled with a third oscillatory circuit, and in order to regulate the band width of the whole filter the damping of the third and last mentioned oscillatory circuit is made variable.

A constructional example of the invention is illustrated in the accompanying drawing. In the filter shown in the figure the two oscillatory circuits Z1 01 and Z2 02 are comparatively closely coupled so that, considered as an ordinary double filter, they possess a resonance curve with two maximum frequency values and an intermediate minimum frequency value. These two oscillatory circuits are preferably so coupled that the coupling is mainly inductive. Naturally the inductive coupling can be replaced by a capacity or galvanic coupling; the direct inductive coupling of the coils Z1 and Z2 has, however, proved to be the most favourable as regards the tuning of the complete filter. The third oscillatory circuit Z3 03 is loosely coupled with the oscillatory circuit Z2 02. This loose coupling can also be achieved in any known manner; a coupling by means of a variable small capacity 04 as illustrated is, however, very advantageous. A resistance wa for varying the damping of the circuit Z3 03 is connected in parallel with the coil and the condenser. The high value ohmic resistance w:

it is preferable to employ a. parallel connected high ohmic resistance. The sets of filter terminals are designated by the reference letters 6 and a respectively. It is immaterial for the operation of the filter if the terminals e or the terminals (1 are taken as the entrance to the filter. The condenser. 03 of the damped oscillatory circuit is preferably made variable, it being expedient to choose the minimum and maximum capacity so that the resonance frequency of Z3 03 can at least be varied within the range between the maximum frequency values Of Z2 02 and Z1 '01.

If it is desired that the range of frequencies passing through the filter should follow a rectangular curve as nearly as possible then 03 is so adjusted that the resonance frequency of la cs coincides approximately with the minimum value of the two-circuit partial filter Z2 c2, 11 01. If the capacity of 04 is small enough it is possible by altering the same to alter the entire basic damping of the filter in such a manner that the basic damping becomes greater the smaller 04 is made. Under these conditions, if resistance .103 is very large the band width will be a minimum, whilst if we is reduced the band width will increase continuously. The maximum band width is ob tained when the damping of l3 c3 attains such a value that the band width of the entire filter becomes approximately equal to that of the twostage partial filter.

If Z3 as is adjusted to be out of tune as regards one of the two maximum frequency values of the two-circuit partial filter, unsymmetrical dampings as regards the mid-point of the frequency range passing through the filter are obtained, the increase of these dampings being readily adjusted by varying we.

A large number of modifications of the described three-part filter are possible without its fundamental properties being affected. It is thus for instance possible to make the coupling between l3 c3 and l2 02 with a low ohmic resistance. The coils Z3 and 12 are in this case each provided near their lower end with a tapping point between which a 10W ohmic conductor is connected. A'

bridge T-element with constant input and out ut resistance but variable transmission factor can then be introduced into this conductor. Furthermore it is also possible to replace the variable condenser 04 by a condenser with a fixed capacity which is located on the sliding contact of a potentiometer connected in parallel with L03. A further modification is possible if the coil 13 is coupled with both of the coils l2 and 11 at the same time, but in this event care must be taken that any alteration in the coupling of la does not alter the coupling between Z2 and Z1 to any appreciable extent.

While preferred illustrative embodiments of the invention have been shown and described, it is to be understood that various modifications in the details of construction and mode of operation may be resorted to without departing from the spirit of the invention Within-the definition of the appended claims.

I claim:

1. A variable width band pass filter comprismeans for adjusting the width of the frequency ing a pair of oscillatory circuits with substane v tially fixed coupling providing maximum transmission values at spaced frequencies separated by. a minimum transmission value at a frequency between said spaced frequencies, a third oscillatory circuit loosely coupled to said pair of circuits and tunable in the frequency range between said spaced frequencies of maximum transmission value, and means for adjusting the width of the frequency band transmitted by said coupled circuits, said means comprising means for adjusting the damping of said third oscillatory circuit.

2.'A variable width band pass filter comprising a pairof oscillatory circuits with substantially fixed coupling providing maximum transmission values at spaced frequencies separated by a minimum transmission value at a frequency between said spaced'frequenciesa third oscillatory circuit loosely coup'led'to said pair of. circuits and tunable over at least the frequency range between said spaced frequencies of'maximum transmission value, and'means for adjusting the width of the frequency band'transmitted by said coupled circuits, said means comprising means for adjusting the damping of'said third oscillatory circuit.

3. A variable width band pass filter comprising a pair of oscillatory circuits with substantially fixed and at least criticalcoupling for transmitting a band of signal. frequencies, a third oscillatory circuit tuned to substantially the average frequency within the band of frequencies transmitted by said pair of oscillatory circuits. means loosely and adjustably couplingsaid third circuit to one circuit of sa'id'pair, and'means for adjusting the width of the frequency band transmitted by said coupled circuits, said means comprising means for adjusting the damping of said third oscillatory circuit.

4. A variable width band pass filter comprising a pair of oscillatory circuits with substantially fixed and at least critical coupling, a third oscillatory circuit, means including a capacitive reactance loosely and adjustably coupling said third circuit to one circuit of said pair, and

band transmitted by said coupled circuits, said means comprising means for adjusting the damping of thethirdoscillatory circuit.

5. A variable Width band pass filter comprising three parallel'tuned oscillatory circuits in cascade in a transmission line, two of said circuits. having a fixed and close coupling and the third circuit being loosely coupled to one of the other circuits, and means for adjusting the width of the frequency band transmitted by said coupled circuits, said, band width adjusting means comprising a variable resistance of high ohmic value connected in parallel with said third oscillatory circuit.

6; In a band pass filter comprising three serially arranged oscillatory circuitswith .atleast one circuit including an adjustable resistance, the method 'of maintaining the intercircuit couplings and the reactances of the several circuits constant while adjusting the band width whichcomprises loosely coupling the circuit that includes the adjustable resistance to another of the circuits, closely coupling said other circuit to thethird circuit, and changing the bandwidth, by changing the value of the said adjustable resistance.

'7. A variable width band pass filter comprising a pair of oscillatory circuits with substantially fixed and at least critical coupling for transmitting a band of signal frequencies, a third oscillatory circuit including means adjustable to tune the same to resonance over a frequency band including one of the frequencies of maximum transmission of said coupled pair of oscillatory circuits, means loosely and adjustably coupling said third circuit to one circuit of said pair, and means'for adjusting the width of the frequency band transmitted by said coupled circuits, said means comprising means for adjusting the damping of said third oscillatory circuit JEAN SCHWARTZ. 

